BELFAST (Reuters) - Britain's plan to trigger Brexit talks by the end of March could be delayed by a political crisis in Northern Ireland if the British Supreme Court rules that Belfast's regional assembly must approve an EU exit, a lawyer for a Brexit challenger said.

Northern Ireland's High Court ruled in October that the province's laws did not restrict British Prime Minister Theresa May's ability to trigger an exit from the European Union, and that the consent of the regional parliament was not required.

But human rights activist Raymond McCord appealed against the ruling to Britain's highest judicial body, which will consider the argument when it rules in the next couple of weeks on whether May can begin the process without the approval of parliament in London.

A likely snap election in the British-run province that may be followed by lengthy renegotiations on the terms of the power-sharing regional government could delay May's plans to begin the talks by the end of March, McCord's lawyer told Reuters.

"In the current circumstances, where there is a potential suspension of the institutions, the approval of the devolved institutions would not be possible," Paul Farrell, a partner at McIvor Farrell Solicitors, said in a telephone interview.

"Devolution has obviously added a layer of complexity to the constitutional arrangements within the United Kingdom and this case is addressing those complex relationships now."

The risk of political paralysis in the region as Britain plans its exit from the EU resulted from Northern Ireland Deputy First Minister Martin McGuinness' resignation on Monday, effectively collapsing the devolved government.

If the British government loses its appeal to the Supreme Court, then May's timetable would come under pressure as lawmakers in London would get a vote on whether she should trigger formal Brexit talks by invoking Article 50 of the Lisbon Treaty.

It is unclear what the Supreme Court's 11 judges will rule on the involvement of regional assemblies such as Northern Ireland's before May can trigger Brexit.

Lawyers told the Supreme Court Northern Ireland's assembly had to give its assent before May could trigger Article 50 and it would be contrary to the 1998 Good Friday agreement, which effectively ended decades of sectarian violence in the province, to begin Brexit without this.

Britain's June 23 vote to leave the EU stirred political tension among the four nations of the United Kingdom - England and Wales, which voted in a majority to leave the EU, and Northern Ireland and Scotland, which voted to remain.

A spokeswoman for May said on Wednesday the timetable for Britain's triggering of Article 50, which kicks off the EU divorce process, was clear, when asked whether anything in Northern Ireland could derail that process.

"There's now this limbo before elections can be called, so we're not going to get ahead of ourselves. We have been clear on the timetable for triggering Article 50 and we will be sticking to that," she said.

(Additional reporting by Michael Holden and William James in London, writing by Padraic Halpin in Dublin; Editing by Guy Faulconbridge and Janet Lawrence)